Scroll fluid machine compresses or expands a working medium by relative movement between scroll bodies including helical wraps. A scroll expander is a type of the scroll fluid machine. The scroll expander includes an expansion chamber formed of a pair of scroll bodies. The scroll expander converts energy upon expansion of a high-pressure working medium in the expansion chamber into rotational energy. As technology in such a field, a scroll expander described in JP 2011-252434 A has been known.